Magnetic field sensors are employed in a wide range of applications including automotive, industrial and consumer applications. Magnetic field sensors can be used in sensors such as proximity sensors, motion sensors, position sensors, speed sensors and angular sensors. Magnetic field sensors include Hall effect sensors and magneto-resistive (XMR) sensors, including sensors such as anisotropic magneto-resistive (AMR) sensors, giant magneto-resistive (GMR) sensors and tunneling magneto-resistive (TMR) sensors.
Magnetic field sensors can be used to obtain speed and position measurements, where the magnetic field sensors detect changes in a magnetic field caused by a moving encoder. Usually, the encoder is an alternating magnetic pole wheel or a ferromagnetic encoder wheel. The alternating magnetic pole wheel is placed next to a magnetic field sensor, which detects polarity changes in the measured magnetic field. The alternating magnetic pole wheel rotates and the magnetic field sensor provides an output signal that indicates a pole has passed by the sensor. The ferromagnetic encoder wheel is placed next to a magnetic field sensor in a standing magnetic field, where the ferromagnetic encoder wheel can be a toothed wheel or a ferromagnetic disc having holes that pass by the magnetic field sensor. The ferromagnetic encoder wheel rotates and deflects the standing magnetic field to create changes in the magnetic field. These changes are measured by the magnetic field sensor via detecting crossings of a switching point between minimum and maximum values.
Often, a magnetic field sensor is calibrated by waiting until a minimum value and a maximum value is obtained via the rotating encoder. A switching point is chosen between these minimum and maximum values. To obtain a switching point before this calibration is complete, a stored value can be read from memory. However, it is difficult to obtain a stored value that yields accurate results over all processes.
Some encoders have regularly spaced segments such as poles, teeth and/or holes and other encoders have irregularly spaced segments. Typically, in the case of a crankshaft sensor system, the encoder includes regularly spaced segments with one exception, such as a missing tooth or a longer magnetic pole. This irregularity indicates a defined position of the crankshaft. From this known position, an angle is calculated by counting the number of edges detected via the magnetic field sensor. The resolution of the system is equal to 360 degrees divided by the number of encoder segments.
To improve resolution, controllers estimate subdivisions by dividing the time between edges into smaller quantities. If the rotational speed of the encoder changes via acceleration or deceleration of the encoder, the estimated subdivisions become less accurate. Corrections can be made by taking into account acceleration or deceleration via comparison of the last segment length and the next to last segment length, i.e. taking the first derivative of the speed, and correcting the estimated subdivisions based on expected changes. Also, higher derivatives of the speed can be taken into account using more and older segment length measurements. However, the age of the derivatives increase with the number of segment lengths used such that the estimated subdivisions are not based on up-to-date information. Also, fabrication tolerances of the encoder lead to different segment lengths and the workload of the controller increases with the complexity of the corrections.
In another effort to improve resolution, manufacturers use more and smaller magnetic poles in the encoder. However, the magnitude of the magnetic field decreases exponentially as the magnetic pole-period decreases, where the magnetic pole-period is one north pole and one south pole in the direction of movement. Thus, as pole-period decreases, the magnetic field decreases exponentially at a given distance from the encoder. This makes it difficult to have a fine resolution encoder and a large air-gap between the encoder and the magnetic field sensor.
For these and other reasons, there is a need for the present invention.